I read Alans comment as available as both service replacement and original equipment.
I contacted the original owner of my 1970 L-78 Chevelle which came out of Missouri.He is sure my car came with the 291 heads and are what is still on it today.My Tonowanda engine build date is Sept.15th 1969.Cars Cowl tag date is Oct.24th 1969
Try the guys at
www.mortec.com
They have some knowledge as well.